The present invention relates to a method for making methylchlorosilanes based on the reaction of particulated silicon and methyl chloride in the presence of a copper-silicon catalyst. More particularly, the present invention relates to the use of particulated silicon which has been contacted with a mixture of partially oxidized copper catalyst and copper formate in the production of methylchlorosilanes.
Prior to the present invention, as shown by Rochow, Chemistry of the Silicones, Second Edition, (1951), John Wiley & Sons, New York Pages 36-46, methylchlorosilanes were made by the direct reaction of particulated silicon and methyl chloride in the presence of metallic copper or silver as a catalyst, while copper chloride or alloys of the aforementioned metal catalysts are often employed. In addition to dimethyldichlorosilane, a variety of other silanes can be formed such as tetramethylsilane, trimethylchlorosilane, methyltrichlorosilane, silicon tetrachloride, trichlorosilane, methyldichlorosilane and dimethylchlorosilane.
In addition to dimethyldichlorosilane, which is the preferred methylchlorosilane of the present invention, "residue" is produced during the formation of the methylchlorosilane crude. Residue means products in the methylchlorosilane crude having a bp &gt;70.degree. C. at atmospheric pressure. Residue consists of such materials as disilanes, for example, symmetrical 1,1,2,2-tetrachlorodimethyldisilane, 1,1,2-trichlorotrimethyldisilane, disiloxanes, disilmethylenes and other higher boiling species, for example, trisilanes, trisiloxanes, trisilmethylene, etc.
Experience has shown that certain components in the residue, as defined above, specifically 1,1,2,2-tetrachloro-dimethyldisilane, and 1,1,2-trichlorotrimethyldisilane can be "cleaved" in accordance with Bluestein, U.S. Pat. No. 2,709,176, French Pat. No. 1,447,304, or Japanese Pat. No. 1,783,419, to produce useful methylchlorosilane monomers. "Cleavability" means the weight percent of the aforementioned disilanes in the residue. Cleavage of the aforementioned disilanes can be effected by using the residue in a continuous stirred tank reactor employing a tertiary organic amine, for example tributylamine with a continuous feed of anhydrous hydrogen chloride.
In addition to making methylchlorosilanes based on the direct reaction between particulated silicon and methyl chloride in the presence of a copper catalyst to produce crude methylchlorosilanes having a reduced weight percent residue based on the total weight of the crude methychlorosilane and cleavability of such residue as previously defined, those skilled in the art also are interested in the T/D ratio of the methylchlorosilane crude. The T/D ratio is the ratio of methyltrichlorosilane to dimethyldichlorosilane in the crude methylchlorosilane reaction product. Accordingly, an increase in the T/D ratio indicates that there is a decrease in the production of the preferred dimethyldichlorosilane.
As shown in copending application Ser. No 288,175, improved results can be obtained in the reaction between particulated silicon and methylchloride by using partially oxidized copper as the copper catalyst. As taught in Ser. No. 288,175, the use of a partially oxidized, or cemented copper catalyst of specific particle-size distribution and exhibiting a minimum surface area (as measured by the BET method) provides an improvement in selectivity with respect to the T/D ratio, based on the reaction between particulated silicon and methylchloride.
The partially oxidized copper catalyst of Ser. No. 288,175 can be made by taking a solution of a copper compound and passing it over scrap iron which results in the deposition of metallic copper in the form of a fine precipitate. The precipitate is then subjected to a pyrometallurgical process which results in the partial oxidation of the cemented copper. There can be in the partially oxidized copper catalyst, a total of 77-87% by weight of copper, which can be in the combined or uncombined form, and preferably 83% by weight. The total reducing power of the partially oxidized copper catalyst is preferably 75-80, while a range of 70-90 can be used. The total reducing power "TRP", the percent cuprous oxide, can be determined by titration with a standard iron sulfate solution using Ferroin indicator.
The metallic copper content of the partially oxidized copper catalyst can vary from 10-20% by weight, and preferably 15-20% by weight of the partially oxidized copper catalyst. The partially oxidized copper catalyst can contain 30-50% by weight of cuprous oxide, while 39-50% by weight of cuprous oxide is preferred. The partially oxidized copper catalyst also can contain 30-50% by weight of cupric oxide, while 35-43% by weight of cupric oxide is preferred.
The partially oxidized copper catalyst utilized in the practice of the present invention can be further characterized as comprising partially oxidized copper particles having a surface area of at least 3.5 m.sup.2 /gram as determined by the Brunauer, Emmett and Teller, Nitrogen Adsorption Method, Jour. Am. Chem. Soc., Vol. 60, p. 309, (1938). In addition, the partially oxidized copper catalyst has a particle size distribution of particles which are less than 35 microns in diameter and greater than 0.7 microns in diameter. Further, 50% of the particles are in the range of from 4-7 microns and the area mean diameter of the particles varies from 3.0-5.5 microns.
Generally, the chloride content of the partially oxidized copper catalyst should be in the range of from about 0-0.2%, and preferably from 0-0.1% by weight and the sulfate content varied from 0-1.5% by weight and preferrably from 0-0.8% by weight. The iron content of the copper catalyst can vary from 0 to 1.5% by weight and preferably from 0-1% by weight. The partially oxidized copper catalyst can have up to about 0.2% by weight of lead and up to about 0.5% by weight of tin, while the water content can vary from 0-0.75% by weight. Other desirable measurements of the partially oxidized copper catalyst are that the apparent density should be from 1.2-1.4 grams per cc and the Fisher number as determined by Fisher Scientific Company Sub Seive Sizer, should vary from 1.8-2.4 micro meters (.mu.m) which is a determination of the air permeability of the powder.
As taught in copending application Ser. No. 288,175, methylchlorosilanes made by the direct method of Rochow utilizing the above-mentioned partially oxidized copper catalyst provide methylchlorosilane mixtures having satisfactory T/D ratio. However, we have found that the weight percent residue of the methylchlorosilane mixtures utilizing the partially oxidized copper catalyst often exceeds acceptable limits.
Additional studies have been conducted to improve the performance of copper as a catalyst for the direct reaction between particulated silicon and organic chloride as shown in Chemistry and Practical Uses of Organosilicon Compounds, V. 1, Leningrad (1958) by P.S. Rostsishevskii, A Study of the Direct Synthesis of Methylchlorosilanes, Institute of Artificial Resins, Warsaw, pages 42-59. Contact masses were used by Rostsishevskii with methyl chloride, which were obtained by heating mixtures of particulated silicon with copper formate.
We have found that the employment of copper formate in place of the partially oxidized copper catalyst in the direct method of Rochow, results in a significant reduction in the weight percent residue produced in the crude methylchlorosilane mixture. However, we have further found that the cleavability of such residue is significantly reduced as compared to the cleavability of the residue provided by the partially oxidized copper catalyst.
The present invention is based on the discovery that a mixture of the partially oxidized copper catalyst and copper formate can provide methylchlorosilane crude having a significant reduction in the weight percent residue, as compared to the use of the partially oxidized catalyst alone. In addition, the cleavability of the residue produced by the use of the mixture of partially oxidized copper catalyst and copper formate is substantially the same as the cleavability of the residue made by using the partially oxidized copper catalyst. Accordingly, a substantial increase in the production of dimethyldichlorosilane and usable methylchlorosilane monomer is provided by the practice of the present invention by a combination of partially oxidized copper catalyst and copper formate. Further improvements such as the batch weight percent of silicon utilization, maximum rate, as well as a substantial enhancement in most situations in the T/D ratio also have been found.